Project Eraser: Reducing complexity in Chrome

Issue description

Project Eraser is the effort to simplify Chrome by unlaunching features and code-paths that no longer provide enough value to justify their complexity either in the code or in the UI. Starting in M-43 we are proactively seeking out opportunities to remove or never launch new functionality on the basis of keeping Chrome code simple and maintainable and Chrome UI easy to understand and intuitive to navigate.
Feel free to cite this bug in discussions/decisions as a decisive reason not to do something (or stop doing something) which is "good" in some sense but doesn't provide enough utility to offset the cost in complexity. Some examples might be:
- Rarely used features that occupy pixels in the UI
- Rarely used codepaths that add complexity to common/important pipelines
- Obsolete or unmaintained stretches of code
- Narrowly differentiated or redundant functionality
- Obscure settings that don't provide significant value
- Anything else where complexity is high and value is low
In other words: https://www.youtube.com/watch?v=PlsW2hd06R0